Where does Reclaim the Net claim Mailchimp is the federal government, or does not have a right to make these decisions? Nowhere: that appears to be a strawman.

It is perfectly legitimate to be angry when companies start restricting their services to political allies, or arbitrarily blocking communication for people who disagree on who to vote for. Freedom of speech is a value encoded in the constitution because it is important, and because societies have learned over and over what happens when it's lost. It's the first thing dictatorships get rid of for a reason.